The present invention relates to a light diffusion sheet, in particular, a light diffusion sheet suitable for use in backlight units of liquid crystal displays.
As light diffusion sheets used for backlight units of liquid crystal displays, there are conventionally used transparent plastic films applied on one surface with a transparent resin solution containing inorganic particles or resin particles dispersed therein.
Performance features required by such light diffusion sheets include invisibility of light diffusion patterns in the light conductive plates, high luminance in the front direction and so forth.
To realize these features, improvements have been made by selecting the type and content of the resin and light diffusion particles used in the light diffusion layers. However, the enhancement of luminance in the front direction that can be achieved by such improvements is limited. Therefore, attempts have been made to direct light in the peripheral direction toward the front direction by means of a prism sheet. Since such a prism sheet does not have light-diffusing ability, the practice has been to superimpose it on a conventional light diffusion sheet. Japanese Patent Unexamined Publication (Kokai) Nos. 9-127314 and 9-197109, for example, disclose light diffusion sheets which, being combined with a lens sheet called a prism sheet, provides enhanced luminance in the front direction compared with conventional light diffusion sheets, together with adequate light-diffusing property.
However, the uneven surfaces of the light diffusion layers of conventional light diffusion sheets are susceptible to damage during handling. Use of damaged light diffusion sheets in today""s high-precision liquid crystal displays is unacceptable because even slight damage to the light diffusion sheet can result in a defective liquid crystal display. Therefore, when the backlight units of liquid crystal displays are produced using these light diffusion sheets, they must be handled with extreme care, and productivity is degraded in proportion.
Accordingly, an object of the present invention is to provide a light diffusion sheet that is high luminance in the front direction, is excellent in light-diffusing property, and is resistant to damage. A further object of the present invention it to provide a light diffusion sheet that is not susceptible to damage when used in a backlight unit of liquid crystal display and can ensure good performance of the liquid crystal display.
The light diffusion sheet of the present invention, which achieves the aforementioned objects, is alight diffusion sheet comprising a light diffusion layer laminated on a transparent substrate and containing a binder resin and resin particles that impart an uneven surface, wherein the uneven surface of the light diffusion layer exhibits an arithmetical mean deviation of 2.0 xcexcm or more in three-dimensional surface roughness measurement.
The light diffusion sheet of the present invention is also a light diffusion sheet comprising a light diffusion layer laminated on a transparent substrate and containing a binder resin and resin particles that impart an uneven surface, wherein the uneven surface of the light diffusion layer exhibits a ten point height of irregularities of 10.0 xcexcm or more in three-dimensional surface roughness measurement.
The arithmetical mean deviation and ten point height of irregularities in three-dimensional surface roughness measurement used with regard to the present invention are values measured by using a surface analysis apparatus (SAS-2010 SAU-II, produced by Meishin Koki Co., Ltd.) according to the methods for measurement of arithmetical mean deviation and ten point height of irregularities for two-dimensional surface roughness defined in JIS-B0601 provided that the methods are expanded to cover three-dimensional surface roughness.
In order to make the light diffusion layer surface resistant to damage, studies have been made so far to improve crosslinking density of the binder of the light diffusion layer and so forth, in particular, to increase hardness of the coated film by using an ionizing radiation hardening resin or the like. However, hardness of the coated film does not necessarily correspond to the resistance to damage on the coated film surface, and the resistance to damage as a light diffusion layer of light diffusion sheet has not been obtained yet hitherto.
Therefore, the inventors of the present invention noted the surface conditions, and assiduously studied. As a result, they found that damages of light diffusion layer surface of light diffusion sheet can be reduced in a degree that had not been obtained so far by controlling three-dimensional surface roughness of the uneven surface of light diffusion layer to be within a specific range, and they accomplished the present invention.
As in the light diffusion sheet of the present invention, when the arithmetical mean deviation of the uneven surface of light diffusion layer is 2.0 xcexcm or more and/or the ten point height of irregularities of the same is 10.0 xcexcm or more in three-dimensional surface roughness measurement, there can be provided a light diffusion sheet of which uneven surface of its light diffusion layer is resistant to damage, while exhibiting excellent luminance and light diffusing properties. In particular, when a backlight unit of liquid crystal display is produced by using the light diffusion sheet of the present invention, the uneven surface of the light diffusion layer becomes not susceptible to damage during handling. Such a light diffusion sheet can easily be handled even if it is large in size, and, therefore, it is extremely effective for the production of the large liquid crystal displays.